As advances in microelectronics, microsensors, and wireless communications have occurred, new types of distributed measurement systems have been proposed and, in some cases, implemented. It is possible to implement such measurement systems by appropriately implementing the measurement functionality and communication functionality of the sensor devices. In general, the sensor-devices are designed to operate over extended periods using battery power and/or passively generated power (e.g., photo-voltaic resources). Also, the sensor devices generally are designed within relative minimal complexity (e.g., limited computational, memory and communication resources). Also, the sensor devices of these systems communicate using short-range wireless methods. For example, ad hoc wireless networks (e.g., IEEE 802.11b networks, Bluetooth networks, and/or the like) may be formed by the sensor devices to facilitate the transfer of measurement data. The organization of sensor-devices using short-range wireless communication protocols are referred to as scatter nets, ad hoc sensor nets, pico nets, and/or the like.
FIG. 1 depicts a typical distributed sensor system 100 that employs a plurality of sensor devices 102. Sensor system 100 could be used to gather measurements for any number of applications. For example, sensor system 100 could be used to obtain chemical measurements across a city to facilitate environmental monitoring of the city. Depending on the intended purpose of sensor system 100, the number of sensor devices 102 within the system may range from a handful of sensor devices 102 to thousands or more. Using higher-powered radio communications with access points 103, collection point devices 101 enable the measurement data to be forwarded to application server 105 through network 104. Application server 105 processes the data according to higher-level algorithms as appropriate for the intended purpose of sensor system 100.
Within distributed sensor system 100, sensor devices 102 are organized in respective scatter nets (shown as nets 106-1 and 106-2). As shown in FIG. 1, sensors 102-1, 102-2, and 102-3 communicate with collection point device 101-1 thereby forming net 106-1. Likewise, sensors 102-4, 102-5, and 102-6 communicate with collection point device 101-2 thereby forming net 106-2. Within their respective net 106, sensors 102 utilize low-energy short-range radio communication to forward the processed measurement data to a respective collection point device 101.
The communication between an individual sensor 102 and a respective collection point device 101 need not be direct. For example, sensor 102-3 may forward measurement data to sensor 102-2 which will then forward the data to collection point device 101-1. When a relatively large number of sensor devices 102 are employed within a respective scatter net, the number of communications “hops” to collection point device 101 can be significant. The number of communications hops is related to the amount of energy required to reach the collection point. Accordingly, the selection of nodes to perform collection point services affects the total amount of energy expended by the sensor net.
A number of protocols exist that attempt to minimize the amount of energy expended by the sensor net by judiciously selecting nodes to perform collection point services. Two such protocols are the LEACH protocol and the PEGASIS protocol. These protocols are based on a number of assumptions. First, these protocols assume that conservation of power is a dominant issue since the sensor devices are battery powered and intended to be deployed in the field for a relatively long time. Also, these protocols assume that all of the sensor samples are important and must be delivered to access points. The protocols assume that access to the external world is fixed through static access points 103. Furthermore, the protocols also assume that the communication with access points 103 occurs according to defined periods.
As a result of these assumptions, nodes are selected to perform collection point services at random. Data is then routed from individual sensors to the nearest respective collection point node 101. Data is aggregated in transit to the collection point nodes 101 when possible. Non-collection point nodes operate at power levels that are only sufficient to reach the “next” node in route to a collection point node. Collection point nodes 101 operate at considerably higher power to transmit over the longer distance to an access point 103. Because collection point nodes 101 consume more power than other nodes, the selection of collection point nodes 101 occurs repetitively to average the energy dissipation patterns over the entire sensor net. Additionally, the transmission of data from sensors 102 to collection points 101 and from collection points 101 to access points 103 are carefully planned to enable nodes to operate in a low power sleep mode most of the time.